Category: Cisco routing

Overview

VRF which stands for virtual routing and forwarding is a feature that allows a single physical router to segregate network traffic into separate virtual routing instances. A router has only one global routing table by default which includes all connected, static and dynamic learned routes. As an analogy VRFs can be viewed as a segregation feature at layer 3 similar to VLANs at layer 2. If we take the example of an ISP with multiple customers by using VRFs we can have separate routing tables per customer on the same router. VRF-lite is a simple form of VRF implementation which is used without MPLS and is appropriate for small to medium enterprises. Being logically isolated each VRF can use overlapping IP address spaces without conflicting with each other.

Overview

Generic routing encapsulation (GRE) is a tunneling protocol which was initially developed by Cisco, and later it has been adopted as an industry standard in RFC 2784. GRE allows the encapsulation of a wide variety of network layer protocols inside virtual point-to-point links. This means that the original packet is encapsulated inside a GRE header and a new IP header containing the source and the destination of the tunnel endpoints. The GRE protocol does not provide any security for the data being transported so if encryption is needed GRE must be used in conjunction with IPsec protocol. Some of the reasons for using GRE are the need to transport multicast traffic, or to provide workarounds for networks with limited hops. In this article we will demonstrate how two networks which do not have reachability can be connected through an GRE tunnel.

Overview

Normally when a routing device receives a packet it decides where to forward it based on the destination address of the packet. Policy Based Routing (PBR) is a mechanism which allows you forward packets based on policies manually defined by network administrators. A good use case for PBR is when a company which has multiple outside connections to different ISPs needs to control how traffic can be distributed across these connections. Compared to traditional routing PBR allows you to implement routing policies based on different criterias like source or destination address, source or destination port, protocol, size of the packet, packet classification and so on. Cisco introduced this feature on Cisco ASA beginning with version 9.4(1). Let’s dive into the PBR configuration.